1. Field of the Invention
The present invention relates generally to pressure-grouted foundation pile forming equipment, and in particular to an auger bit adapted for substantially fully displacing soil while drilling a borehole, and a corresponding pile forming method using same.
2. Description of the Related Art
In the field of foundation construction, pile-type foundation systems are commonly used to support a wide variety of structures. Typical structural applications include commercial buildings, institutional buildings, industrial facilities, power plants, transportation and other structures involving relatively heavy static loads. Moreover, dynamic loads associated with operating equipment can be accommodated by pile-type foundation systems.
The piles comprising such foundation systems can be formed with poured-in-place concrete, which is generally poured into predrilled boreholes around steel reinforcing bar cages, which have been preset in the boreholes. Auger pressure grouting (“APG”) represents another type of pile forming technique wherein grout (generally comprising cement, fine aggregate, such as sand, and appropriate admixtures) is injected under pressure through the auger bit into the borehole, for example, during the extraction of the auger bit. APG foundations generally offer advantages of relatively high bearing capacities and relatively fast, cost-effective construction. Moreover, significant material savings can often be achieved, as compared to comparable poured-in-place pile foundation systems.
Auger pressure grouting with displacement (“APGD”) methods can offer further advantages, particularly with respect to the elimination of excessive spoilage extracted from the boreholes, which presents a disposal problem. Spoilage disposal can be particularly expensive and problematical when hazardous wastes are encountered in the subsurface soil being drilled, for example in environmental remediation projects and on project sites containing buried hazardous wastes. An APGD pile forming apparatus is shown in U.S. Pat. No. 6,033,152. The auger bit shown therein includes a lower section with constant-diameter, right-hand flighting on a downwardly-tapered core and an upper section with reverse (left-hand) flighting on an upwardly-tapered core. The tapered configuration of the lower section tends to displace and compact the soil laterally. The reverse flighting of the upper section pushes the spoilage brought up by the lower section back downwardly and outwardly for compaction.
Several benefits can be achieved with such displacement. The lateral displacement tends to “improve” the soil. Specifically, the borehole is thus lined with compacted soil, which tends to contain the grout and prevent its dispersal into loose, uncompacted surrounding soil. Another benefit relates to minimizing the quantity of spoilage exiting the borehole at grade. As compared to conventional, full-flight augers, displacement-type auger bits tend to displace soil capable of displacement. Displacement also avoids the extraction of soft, sloppy, water-laden soil. Another disadvantage associated with conventional, full-flight augers relates to over-excavation whereby excessive quantities of softer soil are extracted from certain portions of boreholes. The resulting over-excavated boreholes often have hourglass-shaped configurations with enlarged portions, which tend to require excessive quantities of grout or concrete, as compared to cylindrical, straight-walled boreholes. Such extra material can be relatively expensive, particularly when multiple and relatively deep boreholes are affected.
Lateral soil displacement can be accomplished with auger bits having tapered stems, which tend to force the displaced soil laterally outwardly. An example is shown in U.S. Pat. No. 6,033,152, which discloses a “full” displacement auger bit with a tapered stem and bidirectional flighting. The stem expands upwardly from a minimum diameter at its lower end to a maximum diameter at a transition section where the flighting reverses, and contracts back to a reduced diameter at an upper end of the auger bit. The flighting has a relatively constant diameter, which is approximately equal to the maximum diameter of the stem at the transition section whereby substantially “full” displacement occurs at the transition section. The fully-expanded stem and the bidirectional flighting of this auger bit cooperate to force substantially all of the displaced soil to the transition section of the bit, which “displaces” and compacts it laterally into the borehole periphery. The borehole periphery is thereby “improved”, with greater grout-retaining capacity.
Pile forming operations can extend to considerable depths, as required by project structural design criteria and depending upon the load-bearing capacity of the soil conditions encountered at different depths. For example, APGD piles can extend 50 feet or more into the earth. Pile diameters of two feet or more are relatively common. The various combinations of soil, rock and buried concrete (e.g., from previous projects) encountered in such borings tend to affect the materials and configurations of different cutting tips mounted on the augers. For example, soils with high rock content require bits with special cutting teeth and hardened (e.g., heat-treated) steel construction. Soils comprising primarily clay and/or sand, on the other hand, can be drilled with bits having other tip constructions and configurations.
Wear-resistance is a relatively important aspect of APGD bit design. Costs associated with bit wear and replacement tend to be relatively high. Therefore, minimizing wear and the attendant costs of same are important criteria. The present invention addresses these design criteria. Heretofore there has not been available a full-displacement APGD system and method with the advantages and features of the present invention.